Property regulations of binary alkali carbonates by SiO2 nanoparticles for high-temperature thermal energy storage

Abstract

Affordable molten carbonate salts exhibit potential as heat transfer fluids and thermal storage media for the next-generation concentrating solar plants. However, the limited thermal conductivity and specific heat capacity limit their applications at high temperatures. In this work, the carbonate salt-based nanofluids comprising a binary carbonate salt (50 mol.% Na₂CO₃, 50 mol.% K₂CO₃) and varying fractions of SiO₂ nanoparticles were developed for thermal energy storage. Molecular dynamics simulations were utilized in calculating the thermal conductivity and specific heat capacity within the temperature range of 1200–1700 K, concentrating on the effects of nanoparticle fractions. Results indicate that as the volume fraction increases from 1 % to 8 % (defined at 1200 K), specific heat capacity decreases by 0.24–0.83 %, while thermal conductivity improves by 9.7–11.8 %. Subsequent analyses of microstructural evolution, thermal diffusion characteristics, and energy density distribution elucidate the influencing mechanisms of thermal properties. Specifically, interactions between anionic nanoparticle surfaces and salt ions lead to the formation of a condensed interfacial layer encircling the nanoparticle. Within this layer, ions are trapped in a potential well with enhanced order, leading to the layer's high thermal conductivity and specific heat capacity, thereby improving overall thermal properties. Additional analyses of local specific heat capacity and local heat flux confirm that the interfacial layer exhibits higher values than other regions, directly validating the proposed mechanisms. Moreover, the presence of nanoparticles enhances the proportion of energy transport-driven heat flux, particularly within the condensed interfacial layer.